Photophysical properties of [N]phenylenes

In the present study, photophysical properties of [N]phenylenes were studied by means of stationary and time-resolved absorption and fluorescence spectroscopy (in THF at room temperature). For biphenylene (1) and linear [3]phenylene (2a), internal conversion (IC) with quantum yields φ_IC > 0.99 is by far the dominant mechanism of S₁ state deactivation. Angular [3]phenylene (3a), the zig-zag [4]- and [5]phenylenes (3b), (3c), and the triangular [4]phenylene (4) show fluorescence emission with fluorescence quantum yields and lifetimes between φ_F = 0.07 for (3a) and 0.21 for (3c) and τ_F = 20 ns for (3a) and 81 ns for (4). Also, compounds (3) and (4) exhibit triplet formation upon photoexcitation with quantum yields as high as φ_ISC = 0.45 for (3c). The strong differences in the fluorescence properties and in the triplet formation efficiencies between (1) and (2a) on one hand and (3) and (4) on the other are related to the remarkable variation of the internal conversion (IC) rate constants κ_IC. A tentative classification of (1) and (2a) as "fast IC compounds", with κ_IC > 10⁹ s^-1, and of (3) and (4) as "slow IC compounds" with κ_IC ≈ 10⁷ s^-1, is suggested. This classification cannot simply be related to Hückel's rule-type concepts of aromaticity, because the group of "fast IC compounds" consists of "antiaromatic" (1) and "aromatic" (2a), and the group of "slow IC compounds" consists of "antiaromatic" (3b), (4) and "aromatic" (3a), (3c). The IC in the [N]phenylenes is discussed within the framework of the so-called energy gap law established for non-radiative processes in benzenoid hydrocarbons.